John Vine, then chief inspector of borders and immigration, said it was “surprising” fingerprints were not being taken at Calais and other cross-Channel border controls to help identify repeat offenders.

Intelligence and security

The US deploys its formidable intelligence network to keep out extremists and other undesirables, drawing on the surveillance capabilities of agencies including the CIA and the National Security Agency.

In comparison, Britain takes a more softly-softly approach.

The Home Office places heavy emphasis on community-based schemes, as part of the Prevent strategy, to encourage Muslim families, imams and other community leaders to report concerns about potential extremism.

Raffaello Pantucci, of the British think-tank the Royal United Services Institute, said US intelligence agencies take a more “vigorous, pro-active approach”, including extensive surveillance, to track down potential terrorist threats.

“A lot of that is based on surveillance of online activities, which then leads to further action in the offline world,” Mr Pantucci said.

Social media

The US is planning to place social media accounts under intense scrutiny when foreigners apply to visit the States.

Tashfeen Malik and Syed Rizwan Farook photographed on arrival at Chicago’s O’Hare International Airport in July 2014

The day after the San Bernardino attack, it emerged that Malik had posted on Facebook pledging her and Farook's allegiance to the leader of Islamic State, and the couple had also exchanged messages about jihad and martyrdom before they were married.

Richard Burr, a Republican senator who heads the Senate Intelligence Committee, said after the attack he wanted to know how the US immigration authorities “didn't notice the radicalisation".

“In North America, the US particularly, the treatment of immigrants is from the approach that they are potential criminals and potential terrorists.”

Professor Emmanuel Brunet-Jailly

US officials are now working on plans to gather such material in advance to prevent extremists reaching America.

Josh Earnest, the White House spokesman, has announced the Homeland Security and State departments is reviewing the process for screening people who apply for visas.

The Homeland Security Department launched three pilot programmes to carry out social media reviews into its vetting process in 2014.

It is now reviewing policies on the US Citizenship and Immigration Service’s ability to scrutinise social media posts on Twitter, Facebook and other sites as part of the vetting process for visa applicants.

Britain is understood to make very limited use of social media in immigration applications.

Physical security

The US Customs and Border Protection Agency (CBP) has 60,000 employees and describes itself as "America's front line against terrorists".

On an average day it arrests 1,100 people for suspected violations of US laws, including immigration rules.

CBP agents can be heavily armed and are licensed to use "objectively reasonable force”.

US officials are also more likely to use racial profiling – a tactic which is largely prohibited in the UK due to anti-discrimination and human rights laws.

Watch | Boris: ECJ stopping us from controlling our borders

01:39

It is a far cry from the UK Border Force which, although run by a former Second Sea Lord Sir Charles Montgomery, has the staffing and institutional attitude of the civil service.

The UK Border Force has 7,600 staff, and so is one eighth the size of its US counterpart.

The proposal, which was backed by David Cameron as opposition leader, suggested merging police and civilians in a new agency with sweeping police-style powers.

Professor Emmanuel Brunet-Jailly of the University of Victoria, British Columbia, who has researched the US and European approaches to border security, said: "All the infrastructure of the European approach is geared more to integration and human rights of those arriving as migrants.

"In North America, the US particularly, the treatment of immigrants is from the approach that they are potential criminals and potential terrorists."

Visa controls

Anyone who has travelled to Iran, Iraq, Syria and Sudan since 2011 is now exempt from the scheme and will have to apply for a visa, including aid workers and business executives.

The moves will affect citizens of 23 EU countries, including the UK, who had previously been able to enter the US without a visa providing they had been cleared in advance under an electronic authorisation scheme.

The new restrictions also require travellers to the US to possess a passport with embedded security microchips.

'No-fly' lists and terror databases

America also maintains a “no fly” list of people who are to be denied the right to board US-bound flights – which contained 47,000 names according to a 2013 leak.

The US government's main record of suspected international terrorists, the Terrorist Identities Datamart Environment (TIDE), holds more than one million names.

Following the foiled 2009 "underwear bomber" plot the threshold for being place on the terror watch list was lowered.

The change meant that a single credible source became enough for someone to be placed on the watch list.

In December 2014 the Obama administration granted exemptions to broader measures against racial profiling in criminal investigations which it has been argued allowed the practice to continue at airports.

Britain operates comparable databases, principally the Warnings Index, or WI, but the numbers they contain are not known.